docs: def _parse_bracketed_uri_scheme(self):
          """Parse the URI scheme of a bracket-enclosed external link."""
          self._push(contexts.EXT_LINK_URI)
          if self._read() == self._read(1) == "/":
              self._emit_text("//")
              self._head += 2
          else:
              valid = "abcdefghijklmnopqrstuvwxyz0123456789+.-"
              all_valid = lambda: all(char in valid for char in self._read())
              scheme = ""
              while self._read() is not self.END and all_valid():
                  scheme += self._read()
                  self._emit_text(self._read())
                  self._head += 1
              if self._read() != ":":
                  self._fail_route()
              self._emit_text(":")
              self._head += 1
              slashes = self._read() == self._read(1) == "/"
              if slashes:
                  self._emit_text("//")
                  self._head += 2
              if not is_scheme(scheme, slashes):
                  self._fail_route()